Study On Load Shedding Strategy For Power System Under Emergency Situations

The increasing risk of hidden and cascading failures and splitting of power system brings new challenge as the grid structure is more and more complicated. The power shortage threatens in particular in receiving system when the tie line failures and the system instability can happen if the emergency control is limited. Frequency collapse is one of the most important reasons for blackouts in recent years.Combined the subject'the Coordinated Research of Centralized Decision-making and Distributed Implementation in Backup Protection for Large Power Grid'(No.50837002) which is supported by the National Natural Science Foundation of China. This thesis'Study on Load Shedding Strategy for Power System under Emergency Situations' is carried on profoundly. The main innovative results are as follows:Based on the analysis of different power system operation states and the corresponding control methods, system's emergency state is deeply researched. Taking the single machine single load model as the research model, the algebraic differential equation is established based on the system frequency, voltage and power-angle as state variables. Matlab is used to solve the equation of states both in normal and emergency system and to simulate the operation states when system is disturbed by active power and reactive power disturbance. When a large disturbance occurs, it is important to predict and verify whether the system is in the emergency state and thereby to take effective control measures in time, such as load shedding, to prevent the system from instability and to recover both balances of active power and reactive power.By analyzing the dynamic equivalent equation of relating active power disturbance and frequency variation, logic diagram of load frequency control is established in single machine system and multi-machine system with several partitions for further study of frequency response. The dynamic frequency response diagram is clearly established, with the governor, AGC and tie-line bias control considered in the system. The trajectory equation of frequency after the system disturbance is deduced and used in the paper to predict the frequency. It is summarized that at the beginning of the disturbance, frequency change rate is only related to the initial imbalance of power. The frequency decreases faster with a bigger active power shortage and the steady-state frequency deviation is both influenced by the power shortage and frequency regulation factor. The dynamic process of frequency is not only related to the capacity of the active power shortage, but also the disturbance place and power grid's structure. Possessing the function of emergency control, the dynamic frequency response model is built in the paper, which considers system parameters, threshold value in frequency protection and influence on frequency caused by the disturbed power and changed power flow in tie line. By the dynamic frequency response model, the trajectory of frequency and possibility to control the frequency after a disturbance are also analyzed from the mid-term and long-time perspective. Moreover, a new adaptive under frequency load shedding method is proposed in order to apply the dynamic frequency response model to analyze emergency control and system protection dynamically. the load shedding method can be coordinated with under frequency governor control, the reserve capacity and automatic generation control. The new method can calculate the amount of shed load and shedding stage according to system disturbance, thus avoiding shedding over or less. When considering the shedding location, it is necessary to obey the rule that the disturbance power should be balanced locally to avoid transfer of power flow causing cascading accidents.Considering the existing measurement problems, the factors influencing the frequency and its change rate measurement are studied and a precise equation which reflects the relationship between active power imbalance and frequency change rate is presented. Furthermore, the voltage sensitivity is considered to decide the shedding location and coresponding shedding value, aiming to enable the local reactive power balance. It is more reliable to shed the load in the weaker point, namely the node that its voltage or frequency declines faster or needs more reactive power in the disturbance. This method can improve the voltage stability margin and reduce the risk of system collapse. The proposed load shedding method is more adaptive. It is'the functional relation to shedding amount, location and action time, and the change of grid topology is considered, which is more meaningful for system's stability.